1. Field of the Invention
This invention relates to a spinal implant system and method and, more particularly, to an implant having and a lock aid that provides at least one of improved guidance, tactile or visual feedback or enhanced fixation and locking of a cap in a receiver of the implant.
2. Description of the Related Art
Spinal implant screws, such as those used in the pedicles of the thoracic and lumbar spines are commonly used today by spinal surgeons. In a typical prior art screw, there are three main components: a screw shaft, a polyaxial tulip head or receiver incorporating a female thread, and a locking cap incorporating a male thread. The purpose of the threaded locking cap is to affix a spinal stabilizing element, typically a cylindrical rod, to the screw assembly.
During the surgical procedure the surgeon must forcefully seat the rod into the tulip via rotation of the locking cap. Such seating affects the desired correction of the spine. Therefore, the threads of the locking cap and tulip must bear high torque loading which can result in cross-threading and/or thread failure upon initial thread engagement.
Furthermore, the cap may be difficult to engage in the tulip or receiver under such conditions because the thread start-point of both the male and female threads are difficult to identify in a surgical environment. In standard threading, there is little, if any, tactile feedback that the thread is engaged. These problems are exacerbated by the necessity to make thread dimensions very small in order to limit the size of the implant due to anatomical constraints.
Due to such size constraints and strength requirements, a thread with depth greater than thread width is desirable. However, such threads are typically difficult to engage
What is needed, therefore, is a spinal implant incorporating a thread design that prevents cross threading of the small-pitch threads of the locking cap/tulip pair, achieves high connection strength immediately after initial thread engagement and offers improved tactile feedback of thread engagement to the surgeon.
Utilizing blunt-tip threads for the prevention of cross threading was first invented by Higbee in 1891 (U.S. Pat. No. 447,775). Myriad spinal screw designs have been put forth, exemplified by U.S. Pat. No. 5,474,555, however none address the combined issues of threaded cap guidance, visual feedback, and tactile feedback. All of these patents are incorporated herein by reference and made a part hereof.
Some prior art polyaxial screw designs are shown and identified in U.S. Patent Publication Nos. 2007/0043357, 2007/0123867, 2006/0155278, 2007/0123862, 2007/0093827, 2008/0071277, 2008/0097457 and 2008/0249576, all of which are incorporated herein by reference and made a part hereof.
What is needed, therefore, is a system and method that improves the polyaxial screw by providing at least one improved locking aid in the form of at least one of a visual feedback, tactile feedback or enhanced fixations or locking of the cap on the tulip.